The invention pertains to a process for washing nitroaromatic products to remove impurities.
In the industrial production of nitrocompounds, such as mononitrobenzene or nitrotoluene, significant amounts of acidic organic by-products are formed. In mononitrobenzene production the main by-product species are nitrophenols (i.e., an organic acid), and in nitrotoluene production they are nitrocresols. Other minor organic by-product impurities are also present. In addition to by-products, other impurities present in the nitrated product are sulfuric acid catalyst and unreacted starting reactants such as benzene, in the mononitrobenzene product, or toluene, in the nitrotoluene product.
The organic acid by-products present in the crude product stream are particularly undesirable since they can adversely affect later users of the products (i.e., use in other processes, such as in the production of aniline in the case of nitrobenzene). The contaminants are therefore typically removed through a series of process steps. These process steps have been described both in the prior art patents and in the literature, e.g., U.S. Pat. No. 6,288,289 Boyd et al.; U.S. Pat. No. 7,326,816 Knauf et al.; U.S. Pat. No. 7,344,650 Knauf et al.; U.S. Pat. No. 7,470,826 Hermann et al.;. K. L. Dunlap, “Nitrobenzene and Nitrotoluenes”, “Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 15”, John Wiley & Sons, Inc., (1981) 916-32; M. Dugal, “Nitrobenzene and Nitrotoluenes”, “Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 17”, John Wiley & Sons, Inc., (2005) on-line; A. A. Guenkel, “Nitrobenzene and Nitrotoluene”, in J. J. McKetta and W. A. Cunningham (Eds.), “Encyclopedia of Chemical Processing and Design”, Marcel Dekker (1990); J.-L. Gustin, “Runaway Reaction Hazards in Processing Organic Nitro Compounds”, Organic Proc. Res. & Dev., 2 (1998) 27-33; H. Hermann et al., “Industrial Nitration of Toluene to Dinitrotoluene”, in L. F. Albright, “Nitration, Recent Laboratory and Industrial Development”, ACS Symposium Series 623, American Chemical Society, Washington, D.C., (1996) 234-249; G. Booth, “Nitro Compounds, Aromatic”, in “Ullmann's Encyclopedia of Industrial Chemistry, 7th Ed.”, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, (2005).
The typical prior art steps for removing product impurities from the stream of nitrated product comprise the steps of water washing, alkaline washing and neutral washing.
The initial step of water washing uses water as the washing liquid and removes inorganic acids, such as sulfuric acid. The acids can be removed with a selectivity that can be adjusted by those knowledgeable in the art to tailor the resultant waste streams (i.e., water waste) and downstream caustic consumption rates with a particular plant's requirements. (The water washing step is sometimes referred to in the art as “acid washing,” because it removes acid. In the present specification, it is referred to as “water washing,” meaning the washing of the product stream with water for the purpose of removing mineral acids.) The corresponding apparatus in which this step is conducted is referred to herein as a “water washer.”
Following the water washing step, organic by-products are extracted from the nitrated organic product by washing it with an aqueous alkaline solution. The base is typically, but is not limited to, sodium hydroxide, sodium carbonate or ammonia. Through this washing step, referred to herein as “alkaline washing,” the acidic organic by-products, which are dissolved in the organic product phase, are neutralized by the base and to converted to organic salts, which readily transfer into the aqueous washing solution. To achieve industry-accepted product specifications for the acidic organic by-products, more than one stage of alkaline washing is typically used. The corresponding apparatus in which this washing step is conducted is referred to herein as an “alkaline washer.”
The above-described washing steps are carried out by mixing the two immiscible fluids together to transfer the target compounds from one phase to the other, followed by settling of the mixture back into two phases to allow separation and recovery of the two fluids. These can be single or multiple units, where multiple units can be arranged in cross-flow or more commonly a counter-current flow pattern and some degree of recycling/recirculating of the wash solutions, within each unit, is typically practiced.
The organic product leaving the alkaline washing step typically carries with it a small amount of the base (e.g., sodium hydroxide) used in the extraction, and a small amount of the salt formed in the alkaline washer (e.g., sodium nitrophenolates in the case of nitrobenzene production). More specifically, this residual salt is carried by small water droplets entrained in the organic product leaving the alkaline washing step, rather than by the organic product itself. As discussed below, this entrained salt, if not properly removed, can present a significant challenge in the operation of the downstream equipment. To minimize the effect of salt carry-over, one or more “neutral washers” (i.e., salt-removal washers) downstream of the alkaline washers are used. In this specification, the term “neutral washing” refers to washing the nitrated product stream with water to remove salts. The water has a substantially neutral pH. Multiple units can be used, operated in cross-flow or counter-current-flow arrangements. The wastewater from these units can be introduced into the upstream alkaline washing step to recycle the recovered alkaline salt.
Having removed inorganic acids (i.e., in the water washer), organic acids (i.e., in the alkaline washer) and hydroxyl-nitro-aromatics (i.e., in the neutral washer), the next step is to remove residual organic reactant. Some of the reactions to produce nitroaromatics are run with an excess of the organic feed reactant. For the example of mononitrobenzene, excess benzene is used, which remains in the crude product stream. Therefore, the product leaving the washing train is typically sent, directly or indirectly, to either a stripper or a distillation column to recover the excess organic reactant, which up to this point in the process remains in solution with the nitrated product.
In a live steam stripper, the excess organic feed reactant is stripped and removed through the top of the column, then condensed and recycled back to the process reactor. The nitrated product leaves the bottom of the column together with any steam condensed in the column. Within the column, entrained caustic, or other salts, in the nitrated product fed to the stripper is transferred to the water condensate. Outside the column, the nitrated product is separated from the condensate and sent to the downstream process plant. In the case of the mononitrobenzene process, the downstream process plant would typically be aniline production. The product mononitrobenzene leaving the plant typically carries with it a small amount of entrained water condensate which contains some of the sodium hydroxide, or salts, present in the feed to the stripper. This sodium hydroxide, or salt, eventually ends up in the downstream aniline process reactor and is suspected of negatively affecting the activity of its catalyst.
A distillation column can also be used instead of a steam stripper to remove excess organic feed reactant from the nitrated product. The main operating difference from steam stripping is that, in a distillation column, heat is introduced indirectly via a reboiler. As a result, no water condensate forms in the column and a “dry” nitrated product is obtained. Without water in the final product, salts that were dissolved in the water entrained with the organic product feed to the column precipitate out, leading to plugging of the column or downstream equipment. Some of this precipitate is carried all the way through with the nitrated product into the downstream process.
Thus it is very important to ensure the removal of salts from the nitrated product before the stripping or distillation steps, underlining the importance of the proper operation of the neutral (salt removal) washing step. In some nitrations, this proper operation becomes critical if very pure product is required, because then the process would typically include a further distillation step downstream of the benzene recovery column. This involves a second distillation column, operating at a higher temperature than the column for the removal of excess reactant, where heavy components are distilled out. In such a case, un-removed residual salts can lead to chemical instabilities, within the column, with hazardous results.
In general, properly designed neutral washers are very effective in removing entrained salt from the nitrated product. However, neutral washers are also operationally sensitive. This sensitivity is exhibited by a tendency of the two phases (i.e., water and nitrated product) in the washing operation to form one relatively emulsified phase that does not properly settle out into the two phases. The resulting effect is that excessive water can be carried over into the downstream unit operations, which can lead to production shutdowns. This formation, of a single emulsified phase, can occur if the operation on the washer/separator is allowed to drift out of design conditions (e.g., flow rates, mixing intensity, temperature, etc.). It also tends to occur more frequently as one pushes the neutral washer to ever cleaner product, for example by using more than one neutral washer in counter-current flow mode or a larger flow of water in a single neutral washer. Either one of the latter two conditions is desired to achieve significant salt extraction but they are typically not practical without costly separation enhancing equipment such as electrophoresis or coalescers.
Even when good separation of the two fluids (i.e., water and nitrated product) occurs within the neutral washer, whether or not enhanced by for example a coalescer, there is still a significant amount of water entrainment with the exiting product, visible by the cloudy or milky appearance of the nitrated product. This appearance is due to water present, as a “colloidal” stable form, in the product. As noted above, it is the entrained water that carries the bulk of the salt entrained in the nitrated product leaving the neutral washing stage. This concentration of “colloidal” water droplets in the product cannot be easily decreased under the typical operating conditions of the neutral washer. This field observation (i.e., the existence of a minimum achievable water entrainment) has been confirmed in to laboratory experiments. This entrained colloidal water typically amounts to around 1 wt % of the nitrated product leaving a neutral washer, and contains a residual fraction of the dissolved salts in the water used in the washing step. The latter issue leads to design and operating constraints, for example, the provision for excess equipment capacity installation to is achieve desired salt level in the nitrated product.